In blasting operations, such as quarry blasting or mining operations, a plurality of small diameter bore holes are typically drilled in a predetermined pattern to effect a desired fragmentation pattern. Each bore hole in the pattern contains a detonating device for detonating a column of a bulk explosive composition. Each detonating device is interconnected to an initiation system by a signal transmission device for initiating the explosives.
It is desirable that the initiation system used to initiate the bore hole detonators be simple, quick and safe to set up and operate. It is also desirable that, during initiation of a bore hole detonator, the signal transmission device used to transmit the initiation signal not disturb the column of bulk explosive in any individual bore hole so as to cause premature explosive ignition or other unwanted disturbances. After the blasting initiation sequence is completed, it is desirable that any remaining initiation system debris be consumed during the blasting sequence to minimize removal problems.
There are a number of signal transmission devices available for use in an initiation system which posses some of the desirable qualities described herein before including both detonating cord and shock tube; however, no signal transmission device alone possess all of the aforementioned desirable qualities.
The term detonating cord includes a flexible cord made of various combinations of textile and plastic wrappings and containing a high velocity explosive, such as PETN, in the core. The wrapping material provides the cord's tensile strength and water resistence, and is consumed during signal propagation (detonation of the cord). A problem with detonating cord initiation of a bore hole detonator is that the detonation of the cord during signal propagation may produce unwanted disturbances of the column of explosive in the bore hole.
The shock tube may be of the type disclosed in U.S. Pat. No. 3,590,739, sold under the trademark "Nonel". As used herein, the term shock tube refers to any detonating or deflagrating transmission device including a flexible hollow tube which can carry a signal along its interior, which signal does not destroy the tube. A problem with an initiation system using only shock tube initiation of a bore hole detonator is that the shock tube above the surface of the bore hole explosive is not consumed during the blasting sequence, and therefor must often be removed from the production of the blast, such a debris removal problem being particularly important in, for example, salt mining.
It is therefor a primary object of the invention to provide an improved signal transmission device for use in a blasting initiation system which device does not disturb a column of explosive during signal transmission past the column of explosive to the bottom of the bore hole and which device is completely consumed after completion of a blasting sequence; it is another object of the invention to provide a pre-assembled connection between discrete lengths of signal transmission devices for reliable signal propagation between the discrete lengths of transmission devices; it is a further object of the invention to provide a low cost, reliable and high tensile strength connection between discrete length of transmission devices; it is another object of the invention to provide a connection between discrete lenghts of transmission devices which is resistant to penetration by environmental contaminants; it is a still further object of the invention to provide a factory assembled connection between a detonating cord and a shock tube for reliable signal transmission therebetween.
Other objects will be in part obvious and in part pointed out in more detail hereinafter.
A better understanding of the objects, advantages, features, properties and relations of the invention will be obtained from the following description and accompanying drawings which set forth certain illustrative embodiments and are indicative of the various ways in which the principles of the invention are employed.